InfraredInfrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves...
and thermal testing is one of many
Nondestructive testingNondestructive testing is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage. Because NDT does not permanently alter the article being inspected, it is a highly-valuable technique that can save both...
techniques designated by the American Society of Nondestructive Testing (ASNT). Infrared Thermography is the
scienceScience is in its broadest sense to any systematic knowledge-base or prescriptive practice that is capable of resulting in a prediction or predictable type of outcome...
of measuring and mapping surface temperatures.
"Infrared thermography, a nondestructive, remote sensing technique, has proved to be an effective, convenient, and economical method of testing concrete. It can detect internal voids, delaminations, and cracks in concrete structures such as bridgeA bridge is a structure built to span a valley, road, body of water, or other physical obstacle, for the purpose of providing passage over the obstacle. Designs of bridges vary depending on the function of the bridge and the nature of the terrain where the bridge is constructed.-History:The first...
decks, highwayA highway is a main road for travel by the public between important destinations, such as cities and states. Highway designs vary widely and can range from a two-lane road without margins to a multi-lane, grade separated motorway. In English and U.S...
pavements, garage floors, parking lot pavements, and building walls.
InfraredInfrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves...
and thermal testing is one of many
Nondestructive testingNondestructive testing is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage. Because NDT does not permanently alter the article being inspected, it is a highly-valuable technique that can save both...
techniques designated by the American Society of Nondestructive Testing (ASNT). Infrared Thermography is the
scienceScience is in its broadest sense to any systematic knowledge-base or prescriptive practice that is capable of resulting in a prediction or predictable type of outcome...
of measuring and mapping surface temperatures.
"Infrared thermography, a nondestructive, remote sensing technique, has proved to be an effective, convenient, and economical method of testing concrete. It can detect internal voids, delaminations, and cracks in concrete structures such as bridgeA bridge is a structure built to span a valley, road, body of water, or other physical obstacle, for the purpose of providing passage over the obstacle. Designs of bridges vary depending on the function of the bridge and the nature of the terrain where the bridge is constructed.-History:The first...
decks, highwayA highway is a main road for travel by the public between important destinations, such as cities and states. Highway designs vary widely and can range from a two-lane road without margins to a multi-lane, grade separated motorway. In English and U.S...
pavements, garage floors, parking lot pavements, and building walls. As a testing technique, some of its most important qualities are that (1) it is accurate; (2) it is repeatable; (3) it need not inconvenience the public; and (4) it is economical."
An infrared thermographic scanning system can measure and view temperature patterns based upon temperature differences as small as a few hundredths of a degree Celsius. Infrared thermographic testing may be performed during day or night, depending on environmental conditions and the desired results.
All objects emit
electromagnetic radiationElectromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation...
of a
wavelengthIn physics, the wavelength of a sinusoidal wave is the spatial period of the wave – the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
dependent on the object’s temperature. The frequency of the radiation is inversely proportional to the temperature. In infrared thermography, the radiation is detected and measured with infrared imagers (radiometers). The imagers contain an infrared detector that converts the emitting radiation into electrical signals that are displayed on a color or black & white computer display monitor.
A typical application for regularly available IR Thermographic equipment is looking for “hot spots” in electrical equipment, which illustrates high resistance areas in electrical circuits. These “hot spots” are usually measured in the range of 40 °C to 150 °C (70 to 270 °F) above ambient temperatures. But, when engineers use its patented proprietary systems to locate subsurface targets such as Underground Storage Tanks (USTs),
pipelinesPipeline transport is the transportation of goods through a pipe. Most commonly, liquid and gases are sent, but pneumatic tubes that transport solid capsules using compressed air have also been used....
, pipeline leaks and their plumes, and in this project, hidden tunnels, we are looking for temperature patterns typically in the range of 0.01 °C to 1 °C above or below ambient temperatures.
After the thermal data is processed, it can be displayed on a monitor in multiple shades of gray scale or color. The colors displayed on the thermogram are arbitrarily set by the Thermographer to best illustrate the infrared data being analyzed.
In this roofing investigation application, infrared thermographic data was collected during daytime hours, on both sunny and rainy days. This data collection time allowed for
solar heatingSolar heating is the usage of solar energy to provide process, space or water heating. The heating of water is covered in solar hot water. Solar heating design is divided into two groups:...
of the roof, and any entrapped water within the roofing system, during the daylight hours. IR data was observed until the roof had sufficiently warmed to allow detection of the entrapped wet areas because of their ability to collect and store more heat than the dry insulated areas. The wet areas would also transfer the heat at a faster rate than the dry insulated roof areas. At this point in time, the wet areas showed up as warmer roof surface temperatures than the surrounding dry background areas of the roof.
During the rainy day, with minimum solar loading, any entrapped leak plumes would become evident because of their cooler temperature as compared to the dry roof areas
An infrared thermographic scanning system measures surface temperatures only. But the surface temperatures that are measured on the surface of the ground, above a buried pipeline, are, to a great extent, dependent upon the subsurface conditions.
The subsurface configuration effects are based upon the theory that energy cannot be stopped from flowing from warmer to cooler areas, it can only be slowed down by the insulating effects of the material through which it is flowing. Various types of construction materials have different insulating abilities. In addition, differing types of pipeline defects have different insulating values.
Background
There are three ways of transferring energy: 1)
conductionIn heat transfer, conduction is the transfer of thermal energy between neighboring molecules in a substance due to a temperature gradient. It always takes place from a region of higher temperature to a region of lower temperature, and acts to equalize temperature differences. Conduction takes...
; 2)
convectionConvection is the movement of molecules within fluids . Convection is one of the major modes of heat transfer and mass transfer...
; and 3)
radiationIn physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body...
. Good solid backfill should have the least resistance to conduction of energy and the convection gas radiation effects should be negligible. The various types of problems associated with soil erosion and poor backfill surrounding buried pipelines increase the insulating ability of the soil, by reducing the energy conduction properties, without substantially increasing the convection effects. This is because dead air spaces do not allow the formation of convection currents.
In order to have an energy flow, there must be an energy source. Since buried pipeline testing can involve large areas, the heat source has to be low cost and able to give the ground surface above the pipeline an even distribution of heat. The sun fulfills both of these requirements. The ground surface reacts, storing or transmitting the energy received.
Pipeline testing
For
pipelinesPipeline transport is the transportation of goods through a pipe. Most commonly, liquid and gases are sent, but pneumatic tubes that transport solid capsules using compressed air have also been used....
carrying fluids at temperatures above or below the ambient ground temperatures (i.e., steam, oil, liquefied gases, or chemicals), an alternative is to use the heat sinking ability of the earth to draw heat from the pipeline under test. The crucial point to remember is that the energy must be flowing through the ground and fluids.
Ground cover must be evaluated for temperature differentials (i.e., anomalies such as high grass or surface debris), as to how it may effect the surface condition of the test area. Of the three methods of energy transfer, radiation is the method that has the most profound effect upon the ability of the surface to transfer energy. The ability of a material to radiate energy is measured by the emissivity of the material. This is defined as the ability of the material to release energy as compared to a perfect blackbody radiator. This is strictly a surface property. It normally exhibits itself in higher values for rough surfaces and lower values for smooth surfaces. For example, rough concrete may have an emissivity of 0.95 while a shiny piece of tinfoil may have an emissivity of only 0.05. In practical terms, this means that when looking at large areas of ground cover, the engineer in charge of testing must be aware of differing surface textures caused by such things as broom roughed spots, tire rubber tracks, oil spots, loose sand and dirt on the surface and the height of grassy areas.